Corrosion of the steel reinforcement is one of the essential causes for the increased maintenance and repair costs and, consequently, for shortening the useful life of concrete structures. Corrosion of the steel reinforcement is caused by the penetration of chlorides into the concrete cover and/or by carbonisation of the concrete cover. Cathodic corrosion protection (CCP) has been established as a cost-effective, reliable and accepted method of preventing the corrosion of the steel reinforcement. Basically, it is differentiated between two CCP-systems: (1) CCP with external power supply, in which the CCP-system, which consists of an anode installed on the concrete surface or in the concrete structure and the steel reinforcement as cathode, is supplied with direct current from a power supply unit under the control of a control logic. (2) A galvanic CCP without external power supply, in which the anode forms a galvanic element with the steel reinforcement, which galvanic element supplies the current required for the corrosion protection of the steel reinforcement.
In galvanic CCP, the anode acts as a sacrificial anode and is slowly consumed. Usually, zinc and its alloys are used as the anode material. In EP 1 135 538 and in U.S. Pat. No. 4,506,485, a method is described in which the anode material is applied to the concrete surface by flame spraying or by electric-arc spraying. In EP 668 373 A1, a method is described in which a zinc foil, which is coated with an ion-conducting gel, is glued to the concrete surface. Since the anode is consumed during operation of the CCP system, galvanic CCP systems have only a limited useful life. The useful life of such galvanic CCP systems ranges from 10 to 20 years, depending on the amount of zinc applied to the concrete surface and on the locally flowing galvanic currents.
In contrast to the galvanic CCP systems, the useful life of CCP systems with external power supply are only determined by the resistance of the anode to weathering and to the acid development at the anode/concrete interface. Such a system is described, e.g., in EP 1 068 164 B1. CCP systems with external power supply have the advantage over galvanic CCP systems that the current required for the corrosion protection of the steel reinforcement and the voltage applied can be adapted nearly arbitrarily to the requirements of the corrosion protection of the steel reinforcement via the power supply and electronic control units. However, the power supply and electronic control units and the electrical installations (electrical connections to the anodes, to the steel reinforcement and to the monitoring sensors) are expensive, CCP systems using an external power supply require permanent monitoring and maintenance.
In contrast to CCP systems using an external power supply, the voltage applied between anode and cathode in galvanic CCP systems is pre-determined by the galvanic element, e.g. zinc/reinforcement steel. The current flowing between anode and cathode is determined by the electrolytic resistance between anode and cathode and cannot be regulated externally. Therefore, in contrast to CCP systems with external power supply, galvanic CCP systems do not require an external power supply, and the expenditures for the electrical installations, for the electronic control units, maintenance and monitoring are negligibly low as compared to CCP systems using an external power supply.
The electrolytic resistance between anode and cathode is composed of the transition resistance anode/concrete, the electrolytic resistance of the concrete, and the transition resistance concrete/reinforcement steel. As is generally known, the electrolytic resistance of the concrete is highly dependent on the moisture of the concrete and, to a lesser degree, on the chloride content of the concrete. It is characteristic of galvanic CCP systems that products of galvanic oxidation (anodic oxidation) form at the interface anode/concrete, e.g. zinc oxides and zinc hydroxides in case of galvanic zinc anodes. These products increase the electrolytic resistance between anode and cathode in the course of the operation of the galvanic CCP (passivation of the zinc), primarily in a dry environment. This means that galvanic CCP systems require sufficient moisture of the concrete. This is particularly true for galvanic CCP systems in which the anode is applied to the concrete surface by flame spraying or by plasma spraying, as described e.g., in EP 1 135 538 or in U.S. Pat. No. 4,506,485.
For quite some time coatings of zinc or its alloys applied to the concrete surface by thermal spraying have been successfully used for the cathodic corrosion protection of concrete structures in sea water, such as, e.g., bridge components. Since they are close to the sea water, the concrete structures contain sufficient moisture to ensure a flow of current sufficient for the corrosion protection of the steel reinforcement.
In U.S. Pat. No. 6,471,851 and in WO 98/16670, the impregnation of the concrete surface with moisture-retention means before and after the application of the zinc by means of thermal spraying has been suggested. The moisture retention agents, however, do not prevent the formation of passivating zinc oxides and zinc hydroxides which reduce the flow of current to the steel reinforcement. As experience has shown, the moisture retention agents therefore must be repeatedly applied. This involves considerable costs, particularly with construction parts that are difficult to access.
U.S. Pat. No. 6,022,469 describes a galvanic anode system in which the formation of a passivating film is to be prevented by using an alkaline electrolyte between anode and concrete. The electrolyte contains alkaline hydroxide, by which the pH of the electrolyte is adjusted at at least 0.2 units above that pH at which the passivation of the anode occurs. As the person skilled in the art knows, an amount of acid equivalent to the flow of current will form during the operation of the galvanic sacrificial anode in addition to the passivating anodic oxidation products (zinc oxide, zinc hydroxide). This acid neutralizes the alkaline hydroxides in the electrolyte, so that the use of alkaline hydroxides does not constitute a solution for a long-lasting corrosion protection of the steel reinforcement by means of galvanic cathodic corrosion protection.
In EP 0 668 373, a galvanic anode system is described which consists of a zinc foil coated by an ion-conductive elastic pressure-sensitive hydrogel. The hydrogel contains a polymer adhesive, an electrolyte, e.g. sodium chloride dissolved in water, and hydrophilic polymers. The hydrogel-coated zinc foil is glued to the concrete and electrically contacted with the steel reinforcement. This galvanic anode system solves the problem of the variable concrete moisture by arrangement of an ion-conductive hydrogel electrolyte having a high water content (60-95%) between the galvanic zinc anode and the concrete surface. This system, however, has the disadvantage that the adhesion of the anode to the concrete surface is only slight. When moisture penetrates from the concrete or from the sides of the anodes, the hydrogel takes up water, and the adhesion of the zinc foil to the hydrogel and the adhesion of the hydrogel to the surface of the concrete gets lost. Therefore, galvanic zinc-hydrogel anode system can only be used on structural elements which are not exposed to permanent moisture or to a pervasive moisture. The production of the hydrogel and the production of the zinc foil/hydrogel composite material is highly complex and cost-intensive. Experience has shown that the zinc-hydrogel anode has only a very limited shelf life, and after a few months of storage, it can be used only to a very limited extent or not at all for the cathodic corrosion protection of the steel reinforcement.